They will go after the plant the most feeble and tasty. I've never had them on orchids, but on Geranium species only, just like my only meetings with boisduvalii is on Ficus benjamina and not that big an issue. Most pests will go after the plants unable to protect themselves and you never know which ones till you see.
Trouble is, you won't be able to be sure the flies won't go after your orchids…
Another issue is that white flies are resistant to most insecticides.
I've managed to care of them after 2 years of trials when I applied a product with emulsified oil inside (Biplantol contra XL). They are small enough to be affected by the insecticidal oil. But be sure the one you use is well emulsified, or it'll clog the stomates of the leaves.
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